The present invention relates to brakes used on, for example, commercial truck or trailer axles, and in particular to manufacture of brake calipers used in such brakes.
Air disk brakes have been widely adopted in Europe, primarily as result of their performance advantages over conventional drum brakes. These high performance brakes have not been widely adopted in U.S. commercial vehicles. One impediment to wide-scale adoption of disk brakes in the U.S. is the relatively small wheel rims used on U.S. commercial vehicles, as compared those used in Europe, and U.S. vehicle operators' general reluctance to incur the expense of shifting to larger wheel rims, at least in the absence of a regulatory requirement to do so.
The Society of Automotive Engineers is leading work to establish a standard commercial vehicle brake packaging recommendation for industry-wide use, however, this effort has been ongoing for at least eight years without issuance of a formal standard or recommendation, and none is expected to be adopted as a regulatory requirement in the near future (the issuance of a government requirement being seen as a necessity to get commercial vehicle owners to move away from the current standard U.S. wheels). Accordingly, in view of the practical realities of the current commercial vehicle wheel and brake environment, if air disk brakes are to used on a large scale in the U.S. in the near future, air disk brake designs such as those used in Europe must be redesigned to fit within the limited clearance envelope of the existing standard U.S. wheel rim sizes.
Various approaches have been considered for such redesigns, such as using materials other than the usual cast iron, using a smaller diameter brake rotor, and designing the calipers to be thinner in the radial direction to fit within a wheel rim. The common theme among the alternatives is attempting to decrease the radial height of the brake caliper, typically by removing material from the portion of the caliper which bridges over the outer radius of the brake rotor (i.e., the portion of the caliper between the brake application side and the reaction side of the caliper). None of these solutions has yet to provide a design without undesirable compromises, such as prohibitive cost (due to, for example, the use of higher strength, higher cost materials) or insufficient strength and/or fatigue life due to unacceptably thin caliper sections.
Engineering calculations and testing have shown that when a brake caliper is loaded during brake application, there are regions of very high stress in and near the areas of the brake caliper which reach over the outer radius of the brake disk. Calculations have demonstrated that when the amount of cast iron in the cross-disk region of a brake caliper is reduced in order to obtain sufficient wheel rim envelope clearance, the stress levels in cast iron brake calipers manufactured using conventional manufacturing methods are so high as to significantly reduce the fatigue life of the caliper, to the point that adequate caliper life cannot be assured.
Typical approaches to increase fatigue life include increasing the amount of material present in the highly-stressed region; modifying the geometry of the component to further distribute and reduce stresses; moving to higher cost, higher strength materials such as steel alloys, and various surface treatments.
U.S. Pat. No. 5,841,033 shows a method of improving fatigue performance in steel components (a much more ductile material than cast iron, which is brittle and unforgiving of excessive deflection). In this method, a compressive force is applied to specific points along the surface of the components to pre-stress the component in localized areas. This pre-stress is not applied over the entire surface of the components, or to inner regions.
U.S. Pat. No. 4,248,191 shows a pre-stressing method for use in minimizing the potential for cracks in engine cylinder heads. In this method, rings are added in a region between valve seats in the cylinder head. These additional rings are intended to apply compressive forces in the between-seat region, and thereby prevent the occurrence of tensile stresses in the cast iron cylinder head (i.e., if tensile forces are applied to the cylinder head, the pre-stressing ring's compressive forces are intended to be so high that the applied tensile forces never overcome the compressive forces of the rings).
U.S. Pat. No. 5,193,375 shows a method to increase the life of a cast iron brake drum by shot-peening the surface of the brake drum to relieve residual stresses in the surface of the brake drum.
U.S. patent application No. US 2008/0081208 shows an element with a textured surface (i.e., a stamp) which is used to apply a surface pre-stress to a component made of ductile materials such as a stainless nickel-based alloy.
International Patent Publication WO 2004/078275 shows an aluminum or steel golf club head with a pre-determined pre-stress applied to the surface of the club head's striking face, so as to obtain a spring-like effect.
None of these references, however, teaches an approach to improving fatigue life which is applicable to preventing failure of a brake caliper, for example by generation of fatigue cracks, where the highest-stressed material in the caliper is not necessarily located at the surface of the caliper. Moreover, none of the references teach any approach which is compatible with cast iron brake calipers (the foregoing cast iron cylinder head reinforcing ring approach not being relevant to a cast iron brake caliper, as there is no room for the addition of a reinforcing ring in the most highly stressed regions of a brake caliper).